Marcus Tullius Cicero (1872). “The Academic Questions: Treatise De Finibus and Tusculan Disputations of M. R. Cicero, with a Sketch of the Greek Philosophers Mentioned by Cicero”, p.369
Charles Haddon Spurgeon (1988). “Spurgeon at His Best: Over 2200 Striking Quotations from the World's Most Exhaustive and Widely-read Sermon Series”, Baker Publishing Group